Chapter
6: NEMATODESBy Elaine R. Ingham, Oregon State University

THE LIVING SOIL: NEMATODES

Nematodes are non-segmented worms typically 1/500 of an inch
(50 µm) in diameter and 1/20 of an inch (1 mm) in length. Those few species responsible
for plant diseases have received a lot of attention, but far less is known about the
majority of the nematode community that plays beneficial roles in soil.

An incredible variety of nematodes function at several
trophic levels of the soil food web. Some feed on the plants and algae (first trophic
level); others are grazers that feed on bacteria and fungi (second trophic level); and
some feed on other nematodes (higher trophic levels).

Free-living nematodes can be divided into four broad groups
based on their diet. Bacterial-feeders consume bacteria. Fungal-feeders feed
by puncturing the cell wall of fungi and sucking out the internal contents. Predatory
nematodes eat all types of nematodes and protozoa. They eat smaller organisms whole,
or attach themselves to the cuticle of larger nematodes, scraping away until the
preys internal body parts can be extracted. Omnivores eat a variety of
organisms or may have a different diet at each life stage. Root-feeders are plant
parasites, and thus are not free-living in the soil.

Figure 1: Most nematodes in
the soil are not plant parasites. Beneficial nematodes help control disease and cycle
nutrients.Credit: Elaine R. Ingham, Oregon State University, Corvallis

WHAT DO NEMATODES DO?

Nutrient cycling. Like
protozoa, nematodes are important in mineralizing, or releasing, nutrients in
plant-available forms. When nematodes eat bacteria or fungi, ammonium (NH4+) is released
because bacteria and fungi contain much more nitrogen than the nematodes require.

Grazing. At low
nematode densities, feeding by nematodes stimulates the growth rate of prey populations.
That is, bacterial-feeders stimulate bacterial growth, plant-feeders stimulate plant
growth, and so on. At higher densities, nematodes will reduce the population of their
prey. This may decrease plant productivity, may negatively impact mycorrhizal fungi, and
can reduce decomposition and immobilization rates by bacteria and fungi. Predatory
nematodes may regulate populations of bacterial-and fungal-feeding nematodes, thus
preventing over-grazing by those groups. Nematode grazing may control the balance between
bacteria and fungi, and the species composition of the microbial community.

Dispersal of microbes. Nematodes help distribute bacteria and fungi through the soil and along roots by
carrying live and dormant microbes on their surfaces and in their digestive systems.

Food source. Nematodes
are food for higher level predators, including predatory nematodes, soil microarthropods,
and soil insects. They are also parasitized by bacteria and fungi.

Disease suppression
and development. Some nematodes cause disease. Others consume disease-causing
organisms, such as root-feeding nematodes, or prevent their access to roots. These may be
potential biocontrol agents.

Figure 3: Fungal-feeding
nematodes have small, narrow stylets, or spears, in their stoma (mouth) which they use to
puncture thecell walls of fungal hyphae and withdraw the cell fluid. This interaction
releases plant-available nitrogen from fungal biomass.Credit: Elaine R. Ingham, Oregon State University, Corvallis

Figure 5: The Pratylenchus,
or lesion nematode, has a shorter, thicker stylet in its mouth than the root feeder in
Figure 6.Credit: Kathy Merrifield, Oregon State University, Corvallis

Figure 6: Root-feeding nematodes
use their stylets to puncture the thick cell wall of plant root cells and siphon off the
internal contents of the plant cell. This usually causes economically significant damage
to crops. The curved stylet seen inside this nematode is characteristic of the genus Trichodorus.Credit: Elaine R. Ingham, Oregon State University, Corvallis

WHERE ARE NEMATODES?

Nematodes are concentrated near their prey groups.
Bacterial-feeders abound near roots where bacteria congregate; fungal-feeders are near
fungal biomass; root-feeders are concentrated around roots of stressed or susceptible
plants. Predatory nematodes are more likely to be abundant in soils with high numbers of
nematodes.

Because of their size, nematodes tend to be more common in
coarser-textured soils. Nematodes move in water films in large (>1/500 inch or 50 µm)
pore spaces.

Agricultural soils generally support less than 100 nematodes
in each teaspoon (dry gram) of soil. Grasslands may contain 50 to 500 nematodes, and
forest soils generally hold several hundred per teaspoon. The proportion of
bacterial-feeding and fungal-feeding nematodes is related to the amount of bacteria and
fungi in the soil. Commonly, less disturbed soils contain more predatory nematodes,
suggesting that predatory nematodes are highly sensitive to a wide range of disturbances.

NEMATODES AND SOIL QUALITY

Nematodes may be useful indicators of soil quality because of
their tremendous diversity and their participation in many functions at different levels
of the soil food web. Several researchers have proposed approaches to assessing the status
of soil quality by counting the number of nematodes in different families or trophic
groups.* In addition to their diversity, nematodes may be useful indicators because their
populations are relatively stable in response to changes in moisture and temperature (in
contrast to bacteria), yet nematode populations respond to land management changes in
predictable ways. Because they are quite small and live in water films, changes in
nematode populations reflect changes in soil microenvironments.

BUG BIOGRAPHY: Nematode Trappers
One group of fungi may be a useful biological control agent against parasitic nematodes.
These predatory fungi grow through the soil, setting out traps when they detect signs of
their prey. Some species use sticky traps, others make circular rings of hyphae to
constrict their prey. When the trap is set, the fungi put out a lure, attracting nematodes
that are looking for lunch. The nematode, however, becomes lunch for the fungus.